You can't ask "why?" without a language though. And we aren't born with a language besides simple stuff like crying for attention. So, the primal urge is better labeled curiosity, don't you think?
You can't pick one over the other in that case. If humans don't eat they are going to die out. If humans don't have sex they are going to die out. Hunger is not an urge though.. 'to eat' is. So the important urges for survival of the human species are: - eating - having sex - self preservation - motherly care. I guess that pretty much sums it up.
Preserve the code. Please Register or Log in to view the hidden image! And yes, self preservation is a more apt term.
Sleep governs all others. The only thing one needs (and just barely) more than sleep is water. Most people can survive for weeks without food, a few days without water. A few days without sleep and you are on the verge of delusional. Much longer and most people are unable to function effectively.
If someone is attacking me, I will likely kill them to defend myself. Thats a primal urge. If I have to stay up for an exam, I won't kill anyone for it. Sleep is a need, not an urge.